ATLANTA, Georgia (CNN) -- If carb-loading were an Olympic competition, U.S. swimmer Michael Phelps would probably medal there, too. His day starts with three cheese-tomato-onion-fried egg sandwiches, an omelet, three powdered-sugar-covered slices of French toast, a bowl of grits and three chocolate chip pancakes to top it off, according to news reports.

Swimmer Michael Phelps, who set the men's 200-meter freestyle record at the Olympics, packs in the calories.

Phelps told reporters earlier this week he was instructed to eat between 8,000 and 10,000 calories every day. Other news reports put the total as high as 12,000 calories.

This sounds extreme, even to some dietitians. But Olympic athletes' nutritional needs do vary widely according to their sports and body sizes, and swimming for long periods of time will naturally burn a lot of calories, experts told CNN.

"He's a limousine, he's tall. A limousine needs more gas than a Mini Cooper," said Clark, who has worked with Olympic athletes. "Hunger is simply a request for fuel." Watch what Phelps eats »

Different sports need different amounts of fuel, she said. Gymnastics, for example, "doesn't require so much caloric expenditure," she said, and those athletes generally eat less than some others. They also tend to be smaller physically.

Shannon Miller, 31, the most decorated gymnast in U.S. history and winner of seven Olympic medals, told CNN Friday that she didn't have any "off-limits" foods while in training. Her breakfast would be two waffles with butter and syrup, and her working parents would order Chinese food or pizza once a week. Compare her training diet to what Phelps eats »

"I knew I needed to eat in order to have energy, but at the time I really didn't think about it too scientifically," she said. "It was very simple: If I was hungry, I ate."

Miller retired after her medal-winning performance at the 1996 Olympics. She attempted a comeback in the 2000 Olympic trials but injured herself. She will perform in the 2008 Tour of Olympic Superstars.

"After I retired, I immediately hit puberty and it was tough. My entire body changed," she said. "I went from working out over 40 hours a week to nothing, and kept eating the same amount of food. It took some time, but I figured out an 'everything in moderation approach' that works for me."

Still, Miller said she didn't think too much about her eating habits when she was training. She enjoys eating, and had favorite and least favorite foods.

Clark described a typical gymnast's daily menu: a breakfast of cereal, milk and a banana; a lunch of a sandwich and soup; snacks of trail mix, energy bars and fruit; and a dinner of chicken, rice and vegetables.

A weight lifter, Clark said, would eat the same kinds of things, but in larger quantities and with more of a focus on protein. Add eggs and yogurt to the gymnast's breakfast, and multiply the portions of all of the meals -- piles of rice, vegetables and two or three pieces of chicken for dinner.

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Long-distance runners also have high-calorie needs, said Tara Gidus, dietician for the Orlando Magic NBA team and owner of Tara Gidus Nutrition Consulting in Orlando, Florida. She recommends liquids -- smoothies, fruit juices, sweetened juices -- to deliver those calories so that athletes don't have to eat something every hour. Nuts and nut-butters are also dense in energy, she said.

Dinner for the long-distance runner would be carbs -- potatoes, rice, bread -- with some protein -- salmon, chicken, lean beef -- and vegetables mixed in, she said. Antioxidants are key because athletes produce a lot of free radicals, which can result in cell damage.

Gidus discourages athletes from eating foods dripping in fat. But an egg fried in olive oil instead of butter or mayonnaise without saturated fat on a sandwich would be good sources of calories, she said.

Female athletes tend to view food as fattening, while males want to know how they can get more fuel, Clark said. Even at the Olympic level, weight is a big issue among gymnasts, divers and sports runners, she said.